Vaccines that elicit an antigen-specific T cell response to diseased cells harbor great promise (Pavlenko et al., Expert review of vaccines 2005; 4(3):315-327). Because of the inherent risks and high cost of certain vaccines, such as cancer vaccines directed at cells expressing a cancer antigen, there is a great need to identify individuals likely to benefit from cancer vaccination. However, efficient methods for assessing in advance of vaccine administration whether an individual will respond to a particular vaccine were not available prior to the inventors' work.
Cancer vaccines are based on the premise that certain antigens are expressed on cancerous cells but not, or in smaller amounts, on healthy cells. To elicit an immune response against cancerous cells expressing the antigen, the cancer antigen is delivered to an individual, e.g., as protein or nucleic acids encoding a gene for the antigen. The antigen is then processed and presented on antigen-presenting and other cells through MHC class I molecules thereby eliciting a CD8+ cytolytic T cell response directed at cancer cells expressing the antigen (Iwasaki et al., J. Immunol. 1997; 159(1):11-14; Chen et al., J. Immunol. 1998; 160(5):2425-2432)). The immune system of an individual vaccinated with a cancer antigen would, thus, eliminate cancerous cells expressing the antigen.
In practice, administration of an antigen for vaccination does not always follow the process outlined above because antigen exposure can also result in antigen-specific immunological tolerance, i.e., non-responsiveness (Kang et al., 2007; 353(4): 1034-1039; Ruiz et al., J. Immunol. 1999; 162(6):3336-41). For example, in rats, repetitive administration of DNA vaccines is necessary to elicit detectable T-cell responses for an autologous antigen (Johnson, et al., Cancer Immunol. Immunother. 2007; 56(6):885-95). Similarly, multiple immunizations appeared necessary to detect PAP-specific T cells in human patients with prostate cancer (Becker et al, J. Immunol. 2010: (in press)). Likewise, frequent administration of a plasmid DNA encoding myelin basic protein resulted in better clinical outcome for individuals with relapsing-remitting multiple sclerosis, a disease mediated by an inflammatory-type immune response.
The fact that administering an antigen can both elicit or suppress immunity, has raised serious concern. For example, administration of a DNA plasmid for inducing tolerance to self antigens could induce or exacerbate autoimmunity. Conversely, treatment with plasmid DNA for mounting an anti-cancer immune response could result in unwanted tolerance to certain cancer antigens (Johnson el al., Vaccine 2006; 24:293-303). Further, when both an effector and a suppressive immune response is mounted in an individual, the effect of DNA administration to induce immunity might not be phenotypically apparent. For example, during a Phase I clinical trial, several individuals with early recurrent prostate cancer that received six biweekly immunizations with a DNA vaccine encoding prostatic acid phosphatase (PAP) developed PAP-specific effector CD4+ and CD8+ T-cells (McNeel et al. Oncol. 2009; 27(25):4047-4054). In some individuals, these responses were detected shortly after immunization and persisted for several months after immunization. However, in some individuals, a response could not be detected until several months after completing an initial immunization series (Becker et al., J. Immunother. 2010; 33(6) 639-647). The underlying mechanism for these differences in response is unknown.
Delayed-type hypersensitivity (DTH) is a cell-mediated response of the immune system to foreign antigens. For several decades, testing for DTH has been a standard means of evaluating pre-existing cellular immunity to antigens (Hersh et al. Ann. N.Y. Acad. Sci. 1976; 276:386-406). DTH testing is used routinely in the clinical evaluation of prior exposure to agents such as tuberculosis or tetanus, and is associated with other in vitro measures of T-cell immunity, such as T-cell proliferation, Th1-type cytokine bias, and cytolytic T-cells (Gordon et al., J. Allergy Clin. Immunol. 1983; 72(5 Pt 1):487-94, Dietert et al. J. Immunotox. 2008; 5(4):401-12). Loss of DTH response to common antigens has also been used as a standard test for immunosuppression (Huebner et al. Clin. Infect. Dis. 1994; 19(1):26-32).
In recent years, DTH testing has been used to evaluate T-cell responses following immunization with antigen-specific vaccines, particular peptide-based vaccines. For example, a DTH response to immunization with a polyvalent cell-based melanoma vaccine was an independent predictor of clinical outcome (Hsueh et al. J. Clin. Oncol. 1998; 16(9):2913-20). DTH testing has limitations, however, and its use is not always practical or possible. For example, DTH testing of DNA vaccines requires production of separate good manufacturing practice-grade antigens. Also, information derived from the DTH test, i.e., area of induration and erythema, does not necessarily provide mechanistic information because a DTH response can be mediated by multiple factors. DTH responses at the site of immunization do not always indicate an immune response to the administered antigen but, instead, might indicate a response to a vaccine adjuvant, such as the cytokine adjuvant GM-CSF commonly used in anti-cancer antigen vaccines, confounding the interpretation of these results (McNeel et al., Blood 1999; 93(8):2653-9).
DTH can also be assessed trans vivo, i.e., by measuring a swelling response in another organism (vanBuskirk et al., J. Clin. Invest. 2000; 106:145-155). For this trans viva DTH (TV-DTH) assay, peripheral blood mononuclear cells (PBMC) from an individual and an antigen of interest are administered intradermally to an animal, such as into the footpad or ear of a mouse having severe combined immunodeficiency (SCID). The presence of a DTH response is then determined by measuring footpad or ear swelling 24 hours after administration of the cells and the antigen. The TV-DTH assay has been used to evaluate the immune response to target tissue antigens following solid organ transplantation, e.g., to monitor the immune response of renal allograft recipients (Knechtle et al., Am. J. Transpl. 2009; 9:1087), and to investigate mechanisms of tolerance induction (Geissler et al., Transplantation 2001; 72(4):571-80).
Thus, there remains a great need in the art for reliable methods to identify individuals that are likely to benefit from vaccination as well as for methods of improving successful elicitation of a cancer antigen-specific immune response in response to DNA vaccination.